86 Discussion 



depend on them. You can all judge the age of a man, but how do you 

 do it ? One form of tests in rats is adaptation methods : with ageing 

 the power of adaptation, such as to cold, or lack of oxygen, decreases. 

 You can also measure the age of the collagen in the rat's tail tendon 

 (see Verzar, F. (1957). Ciba Found. Coll. Ageing, 3, 60. London: 

 Churchill). But then it turns out that everything ages differently, 

 and rats age differently in their brains than in their tendons. We 

 irradiated rats with 700 r. and they died quickly, but their collagen 

 had not aged. 



Holt: I was going to raise the same point, because my experience 

 is also in the comparison of growth curves which are completed at 

 relatively different rates in different species. The dispersions of the 

 age of incidence curves which you showed. Dr. Berg, in the compari- 

 son between man and rat, seemed proportional to their means. You 

 thought that the curve for man had a higher dispersion because you 

 were dealing in that case with a heterogeneous group ; my interpreta- 

 tion was that both distributions were equally dispersed, because I 

 mentally converted them to equivalent relative time scales. 



Rotblat: Have you ever drawn graphs on which you plot age not in 

 years but in the fraction of the span of life, so that you can compare 

 the spans of life directly ? Otherwise how do you know that the 

 onset of disease is the same in the rat as in man ? 



Berg: Prof. Simms and I have discussed this extensively. He feels 

 that such a plot would be meaningless. The lifespan is determined 

 by the age of onset of lesions. Hence, to use lifespan as a standard 

 for comparing age of onset would have no significance. It would be 

 like comparing the speed of two racehorses — not in terms of minutes 

 per mile — but in terms of minutes per mile multiplied by miles per 

 minute. 



Rotblat: From the change in the slope of the Gompertz curve 

 which you showed us I would expect a large extension of the time 

 scale. 



Tanner: This works out as the equivalent in the human of around 

 17 to 18 years. In other words if you multiply the scale 30 : 1, 

 which is roughly right, the curves you showed for the rat would be 

 almost superimposable on those for man. 



Rotblat: This is what I wanted to know: whether they are really 

 the same if they are superimposed. 



Maynard Smith: To me the most surprising thing you told us, Dr. 

 Berg, was that the ages of onset for a whole variety of at first sight 

 causally unrelated lesions were all shifted in the same direction by the 

 same environmental treatment, i.e. restriction of the diet. I do not 

 believe that one can tell very much about the causes of ageing in any 



